With the term “exhaust gas” it is intended a flue gas that is produced as a result of the combustion of fuels, such gasoline/petrol, diesel, fuel oil or coal. The ever increasing diffusion of power plants, industrial process plants, and motor vehicles in the world, has urgently led to the study of possible solutions for reducing the harmful effects of the exhaust gases on the environment and on the man.
Indeed, although the largest part of most exhaust gases is relatively harmless nitrogen, water vapor (exception made for pure-carbon fuels), and carbon dioxide (with the exception of hydrogen as fuel), a relatively small part thereof is formed by undesirable toxic substances, such as carbon monoxide, hydrocarbons, nitrogen oxides, partly unburnt fuel, and particulate matter. Generally speaking, with the term “particulate matter” (briefly referred to as “PM”) it is intended solid or liquid particles suspended in a gas. In an exhaust gas, such as the exhaust gas produced by a diesel engine, the main fraction of PM is composed of very small particles, mainly consisting of impure carbon particles (in jargon, also referred to as “soot”). Because of their small size, said particles, when inhaled, may easily penetrate deep into the lungs. The rough surfaces of these particles make it easy for them to bind with other toxins in the environment, thus increasing the hazards of particle inhalation. The discharge amount of PM becomes large in a diesel engine using a gas oil as a fuel or a direct-injection type gasoline engine recently coming into wide use.
A solution for removing (or at least reducing) the PM emissions of an exhaust gas produced by fuel combustion, e.g., in a vehicle engine, provides for the use of a particulate filter. Making reference to the exhaust gas produced by a diesel engine, a particulate filter—in this case, referred to as Diesel Particulate Filter (DPF)—is a device arranged in an exhaust gas emission path of the diesel engine for receiving the exhaust gas and retain the PM included thereinto.
A conventional DPF may consist of a cylindrical body made of porous material, such as silicon carbide (SiC), with a first base (upstream side) receiving the flow of the exhaust gas produced by the engine. Such DPF has a honeycomb structure, with a plurality of exhaust gas flowing channels extending in parallel to the longitudinal direction of the cylindrical body, from the upstream side body to a downstream side, corresponding to a second base of the cylindrical opposite to the first one. These channels are alternatively plugged at either the upstream side or the downstream side to form a checker pattern. The exhaust gas (including PM) hits the first surface, and is forced to flow through the channels of the DPF that are not plugged at the upstream side. Thanks to the porosity properties of the SiC, the PM included in the exhaust gas is blocked by the walls of said channels, and remain confined in the DPF, while the rest of the exhaust gas (essentially free of PM) crosses the walls, passes into the adjacent channels and exits from the DPF, for being outputted outside the vehicle through exhaust pipes.
While disposable DPFs exist, the majority of the current DPFs are designed to be subjected to cleaning operations for removing from the DPF the PM accumulated with the use. Particularly, said cleaning operations, also known as filter regeneration operations, may provide for burning off the accumulated PM, providing heat to the accumulated PM in such a way that the latter reaches its burning temperature—which, for a PM made of carbon particles, is around 600-650° C.
According to a first method known in the art, the filter regeneration is performed in a “passive” way, with the DPF that is brought to the burning temperature of the PM by exploiting the heat of the exhaust gas itself. Since however it is very difficult to reach the burning temperature of 600-650° C. simply using the heat of the exhaust gas only, a fuel additive is mixed together with the fuel, which lowers the burning temperature of the PM of about 300° C. According to the latter method, if the temperature of the exhaust gas reaches 300-350° C. and is maintained for a certain time, the PM collected in the DPF burns for spontaneous combustion, emptying the channels thereof. However, even in presence of the fuel additive, bringing the temperature of the exhaust gas to said temperature may be quite difficult, especially for those vehicles equipped with small engines, that frequently run in urban areas at a low speed, lift trucks, operative vehicles that works for long period at the idling condition, and the like. This involves an undesired build-up of PM within the DPF, which even after few hours of operations may occlude the channels of the DPF, with a consequent power off of the vehicle. In view of these reasons, the passive filter regeneration is quite inefficient.
According to a second method known in the art, the filter regeneration is performed in an “active” way, with additional heat that is supplied to the DPF by an external source, to reach the burning temperature of the PM in an easiest way. For example, the DPF may be provided with proper heating devices, that are periodically activated to heat the exhaust gas before it enters into the channels of the DPF. Also according to this solution, the fuel can be mixed with proper fuel additives, for lowering the burning temperature of the PM and facilitating the filter regeneration. For example, the heating devices may be implemented with a spiral-shaped resistance, or with ceramic or metal glow plugs positioned in the proximity of the upstream side of the DPF. Said heating devices are controlled by a suitable control unit in such a way as to heat the exhaust gas at predetermined times for favoring the trigger of the burning of the PM included in the DPF.
The European Patent application EP 990777A1 discloses a regeneration system for an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine. The regeneration system comprises an exhaust gas cleaning honeycomb filter and a heating means for the filter. The filter is a checkered SiC honeycomb filter having a given cell structure, and the heating means is a heater or a glow plug when using a fuel containing fuel additive.
The European Patent application EP 1582714A1 discloses a system and a method for regenerating particulate filters. Particularly, estimated values of an amount of accumulated particulate matter on a particulate filter are obtained before initiation of a forceful regeneration operation of the particulate filter. A maximum operating time period of the regeneration operation is set based on the estimated values of the amount of accumulated particulate matter. The particulate filter is regenerated in the regeneration operation by performing post fuel injection in the diesel engine during each exhaust stroke of the diesel engine to supply fuel to the particulate filter and thereby to remove particulate matter from the particulate filter through use of combustion heat of the supplied fuel upon combustion of the supplied fuel.